CVE-2026-49400: October CMS PHP Object Injection Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-2ff2-mx52-q8wp",
  "modified" : "2026-09-14T17:08:06Z",
  "published" : "2026-09-14T17:08:06Z",
  "aliases" : [ "CVE-2026-49400" ],
  "summary" : "October CMS: PHP Object Injection via Backend Widget Session Storage",
  "details" : "The backend `SessionMaker` trait stored widget session state as `base64(serialize(...))` and consumed it with `unserialize()` without an `allowed_classes` restriction. Any code path that could write to a `widget.*` session key with attacker-controlled bytes could trigger PHP object injection the next time the widget read its session state, allowing instantiation of arbitrary classes and reachable PHP gadget chains.\n\n**This issue only affects installations running with `cms.safe_mode` enabled.** Safe Mode is a niche opt-in feature, primarily used for demo installations and multi-tenant or shared-editor scenarios where untrusted users are deliberately granted access to the CMS markup editor. In standard production deployments Safe Mode is off, backend access is restricted to trusted administrators, and a markup editor can already execute arbitrary PHP directly. The session-write path that reaches this sink is gated by the Safe Mode sandbox, so installations without Safe Mode enabled are not exposed.\n\n**Scope of impact is narrow even with Safe Mode enabled.** The standard backend code paths that populate widget session state (search terms, sort options, selected IDs, filter values) wrap the input inside a known array shape before serializing, so user-supplied values never reach `unserialize()` as a controllable serialized payload. Exploitation requires the Safe Mode session-write path together with a suitable PHP gadget chain reachable from the installed dependency set. The hardening below removes the underlying object-injection sink so the class of issue is closed off regardless.\n\n### Impact\n- Arbitrary PHP code execution as the web server user, via a gadget chain deserialized from a widget session key, triggered the next time the affected widget is rendered\n- Requires `cms.safe_mode` to be enabled, and a backend user with CMS markup editing access who is not intended to be trusted as a full administrator\n- A suitable PHP gadget chain must be reachable from the installed dependency set\n- Not exploitable when Safe Mode is disabled\n\n### Patches\nThe vulnerability has been patched in v3.7.17 and v4.2.21. Two changes were applied:\n\n- `Backend\\Traits\\SessionMaker` now stores widget session state as plain JSON instead of `base64(serialize(...))`, eliminating the object-injection sink entirely for new writes. Reads transparently fall back to the legacy format for one upgrade cycle so existing sessions retain their saved widget state.\n- The legacy `unserialize()` fallback path now sets `allowed_classes => false`, so even values written before the upgrade cannot instantiate objects.\n\n### Workarounds\nIf upgrading immediately is not possible, restrict CMS markup editing access to fully trusted administrators only, the standard October CMS recommendation for any deployment.\n\n### References\n- Reported by EndlssNightmare",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "october/system"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "3.7.17"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "october/system"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "4.0.0"
      }, {
        "fixed" : "4.2.23"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/octobercms/october/security/advisories/GHSA-2ff2-mx52-q8wp"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/octobercms/october"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-502" ],
    "severity" : "LOW",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-14T17:08:06Z",
    "nvd_published_at" : null
  }
}